This work describes the preparation and scintigraphic evaluation of the in vivo distribution patterns in dogs of hydantoins labeled with 20.4 min half-life carbon-11. Carbon-11-labeled HCN was collected in water or aq DMSO containing carrier KCN following bombardment of 99 percent N2 - 1 percent H2 gas mixture with 22 MeV protons for 1 hr at 25-35 micro A. Five C11 5,5-dialkylhydantoins, three C11 diarylhydantoins, five C11 5-alkyl-5-phenylhydantoins, and five C11 spirohydantoins were prepared by heating, generally under pressure, a mixture of C11 KCN, the corresponding aldehyde or ketone, and excess (NH4)2 CO3 in aq ethanol or DMSO solvent. The C11-labeled hydantoins were dissolved in 1-1.5 percent aq NaOH for intravenous administration to dogs. Total synthesis time was 70 to 106 min and 1 to 59 mCi of final product was available for conducting serial in vivo imaging with the gamma scintillation camera. Carbon-11 activity from all compounds showed initial blood-pool distribution with variable concentration of activity in the brain, lungs, liver, and kidney. All of the C11-labeled diarylhydantoins, and most having one phenyl substituent, and one having a hexamethylene moiety showed initial accumulation of C11 activity in brain. Carbon-11-labeled 5,5-diphenylhydantoin (Dilantin) showed the greatest qualitative accumulation of activity in the brain. Those C11-labeled hydantoins having a carboxyl substituent showed prominent renal concentration and urinary excretion of activity. Most C11-labeled hydantoins showed a progressive homogeneous whole body distribution of activity in cellular tissues and slow excretion from the body supports the thesis that the pharmacologic action of the hydantoins is related to physical effects on biomembranes rather than to specific chemical interactions with cell constituents.